Vinyl polymers having a core-shell structure and capable of being used in paint formulations are known. The particles of these polymers are composed of a core and of a shell with a different composition from that of the core. The latex based on these particles is generally obtained by emulsion polymerization. Thus, a description is given in the document WO 95/14063 of an aqueous dispersion of particles composed of a copolymer core obtained by the emulsion polymerization of ethylene and of a vinyl monomer and of a copolymer shell comprising the units of an alkyl (meth)acrylate having a carbon number of between 4 and 12 and of at least one monomer chosen from alkyl (meth)acrylates having a carbon number of between 1 and 3, styrene and .alpha.-methylstyrene.
The document WO 94/20556 discloses a dispersion of particles composed of a polymer core containing one or a number of monomer(s) chosen from vinyl esters, (meth)acrylates, vinyl chloride, aromatic vinyl compounds and ethylene and of a copolymer shell containing units of (meth)acrylates of specific solubility, of a functionalized olefinic monomer and of a hydrolysable unsaturated silane. The core represents 75 to 98% of the total weight of the particle.
Moreover, the document JP-A-3-41103 discloses a method for producing copolymers suitable for the manufacture of plastisols which are stable on storage. Example 5 of this document describes a microsuspension polymerization process, according to which part of the vinyl chloride to be used is introduced before the reaction and the remaining part of the vinyl chloride and the butyl methacrylate are introduced continuously, all the methacrylate being introduced before the degree of conversion of the monomers exceeds 60%. This method exhibits disadvantages illustrated by Comparative Examples 4 and 5. Indeed, it is impossible to obtain stable plastisols with vinyl acetate and when the butyl acrylate is introduced beyond a degree of conversion of the monomers of 60%.